In recent years, it has been proposed to install sensor devices at a variety of indoor and outdoor places, thereby to monitor temperature, humidity, brightness and human movement, and to perform control of lighting and air conditioning on the basis of thus obtained data, and it has already become in practical use in some places. It also has been proposed to perform communication between the sensor devices and a master device, by sending and receiving the obtained data, and then to perform the above-described control in the master device. However, when dealing with events to occur temporally irregularly, sensors (event detection sensors) and transmission/reception means are required to be always in an activated state, and accordingly, power consumption of the event detection sensors and of the sending/reception means have been a problem. In this respect, there has been proposed a sensor system in which a semiconductor switch is started up by a signal of a vibration sensor using the signal as a trigger signal, and the start-up of the semiconductor switch causes a transmission/reception circuit to start up (for example, Patent Literature 1 (PTL1) is referred to). With the technology of Patent Literature 1, power consumption is reduced compared to the case of being always in an activated state, as a result of that both the semiconductor switch and the transmission/reception circuit are in a standby state when no vibration is detected. There also has been proposed a piezoelectric-type vibration sensor in which the vibration sensor is used as a trigger signal means, and thereby a power switch is turned on and off by a vibration at a time of starting operation of the mechanical apparatus (for example, Patent Literature 2 (PTL2) is referred to). In the technology of Patent Literature 2, the vibration sensor is composed of a piezoelectric ceramic plate, a rigid ball and a supporting member on which the rigid ball rolls, and a voltage generated by the rigid ball striking the piezoelectric ceramic plate is used as a detection signal.